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Old 6th November 2006, 05:27 PM   #1
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Default B+ time delay relay

OK, after getting my PSU (more-or-less) designed, I want to add a time-delay relay so that a single switch can be used to turn on the 6.3VAC heater transformer before the B+ transformer gets switched-on.

My thoughts were that a time delay relay, while more expensive would be less stressful to implement. I would use the relay on the AC side of the B+ transformer, and it would be triggered when I switch on the heater transformer. Both switch and relay would be at the AC side of things.

Mouser stock a Magnecraft relay that'll tolerate 120VAC at 12A with a DPDT form that will allow for delays between 0.1 second and 10 hours. Of course, a 10 hour delay would be rather excessive and could result in considerable boredom, although there would be very little listening fatigue

At $36 it is not cheap, but is it likely to work? Does anyone have experience doing this?

Thanks,
Charlie
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Old 6th November 2006, 06:15 PM   #2
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I use two timers in my amp (currently under production).

Both timers are initiated by the presence of voltage, i.e. when the main power switch is turned on. I take control power off the DC bus, which I use for running my filaments. The same approach, however, could be used if you are using AC filaments.

The first relay is set for one minute, and is installed in the return lead (center tap of transformer). Normally, it is off, so there is no B+. When this relay turns on, the B+ charges through a 10 ohm resistor, for limitation of inrush. I tried 100 ohm up to 1000 ohm, but the difference was somewhat trivial, in that either method either charges quickly or will not fully charge. It was not my intent to provide a slow start up, only control inrush.

Second timer is about 5 seconds, and shorts out the 10 ohm resistor. All these components are in the single return lead to the center tap of the transformer. I took this approach so I could guarantee all tubes are sufficiently heated before applying B+, and it also enables me to use my relay contacts in an AC circuit, as opposed to the common method of right in front of the first cap stage. Relay contacts prefer AC over DC, as the zero crossing aids interruption.

Sure, I could use a NTC or something, but I liked the idea of greater control, and ultimately having no resistive element in the B+ supply during normal operation. It also lets me use very small printed circuit board relays, instead of the larger ice cube style socket relays. It is therefore a little less expensive, and smaller. Small is good, as my amp is bursting at its britches.

I can provide a schematic if you are interested. It has a number of parts, but is compact.
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Old 6th November 2006, 06:27 PM   #3
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I have a question about this as I am also doing the same thing. For the relay delay on the B+ supply, how critical is the voltage rating of the relay? All of them I have seen are rated at voltages much lower than the B+ voltage
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Old 6th November 2006, 07:18 PM   #4
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I will mount the relay before the power transformer. By doing this, the relay will only need to be rated at 120VAC.

Charlie
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Old 7th November 2006, 12:34 AM   #5
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That is certainly one way to approach it, however if one is using the auxiliary windings on the power tranny, you will also not be using those for the initial time delay. The idea was to have all other supplies up (filaments, bias) before the B+ initializes.

With regards to voltage rating of contacts, you need to consult the data sheet. My particular relays were Tyco/Schrack RT2 printed circuit mount. Each contact is rated up to 400VAC, with a dielectric strength of 1000VAC. The 400VAC rating is limited most by interrupting ability, not making ability. If you analyze the operation of the relay, it never interrupts load current. The relay contact opens after there is complete loss of AC power (at least in my design).

So, not only is the voltage rating of little concern, but I place the two contacts in series. Now I do not get exactly twice the voltage rating doing this, but contacts in series is a very common method of increasing contact life in higher voltage circuits. GE has been doing it for years in their power breakers' auxilaries, which need to handle 250VDC at 15A interrupting. Again, this is under the assumption that the contacts need to interrupt load current at rated voltage, something we don't need.

Finally, at the end of the day, the contacts are rated to interrupt 2000VA (that's a lot) and carry 10A continuously, while we require 100-300mA. That's very light duty for a relay. Putting the relay in the 120 side, however, will increase the current requirements.

At the end of the day, I think the most elegant solution is still the tube rectifier, but my preference is not to use them. I think the advantages of SS outweigh the burden of time delay circuits. Or, you could avoid delays altogether, as many believe cathode stripping is a non-issue at levels under 1000V.
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Old 7th November 2006, 11:29 AM   #6
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If you are using a rectifier tube would't it be fairly easy to pop in one that has a longer warm up time? I checked a 5Ar4 against a 5Y3 and it took longer. And there are others.
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Old 7th November 2006, 11:55 AM   #7
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You could also try a 5Z4P as it has a longer warm up time.

Charlie,

Why are you worrying about this anyway? As long as there isn't any signal there shouldn't be a problem. Anyway, this is what I have been told in other threads when I asked about cathode stripping.
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Old 7th November 2006, 05:26 PM   #8
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BurnedFingers,

I will be building a diode-bridge PSU, so the heaters will not get chance to warm before full DC is applied.

It could be that cathode stripping is not going to be a problem, especially as I am going to be using very cheap tubes to begin with. Maybe once I decide to buy better quality tubes, I'll want to nurse them by installing some kind of delay relay.


Regards,
Charlie
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Old 8th November 2006, 03:06 AM   #9
DHT112A is offline DHT112A  United States
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I have used these timers with a relay. Simple and not real expensive. These are not normally available to the public. I am in a business where I can get them through a wholesaler. I am in no way connected with this auction.

http://cgi.ebay.com/ICM102-TIME-DELA...QQcmdZViewItem
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Old 15th November 2006, 08:37 PM   #10
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Default Delay relays

When using two power transformers (one for B+, the other for the heaters), I've found the Amperite 115N060 delay relay tube to work nicely. It comes in a handy octal pinout - looks a bit like a 6SN7 but with more interesting internal structures. You can buy them for ~$15 and they come in 30, 60, 90, and 120 second delays.
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